In magneto-optic recording, data is represented as a magnetized domain on a magnetizable recording medium such as a disk. Each domain is a stable magnetizable data site representative of a data bit. Data is written to the medium by applying a focused beam of high intensity light in the presence of a magnetic field. The disk typically includes a substrate, a magneto-optic recording layer, a reflective layer, and two or more dielectric layers.
In substrate-incident recording, the beam passes through the substrate before it reaches the recording layer. The reflective layer in a substrate-incident recording medium is formed on a side of the recording layer opposite the substrate. The reflective layer reflects the beam back to the recording layer, increasing overall exposure and absorption.
In near-field, air-incident recording, the beam does not pass through the substrate. Instead, the beam is incident on the recording layer from a side of the disk opposite the substrate. In an air-incident recording medium, the reflective layer is formed adjacent the substrate. A solid immersion lens (SIL) can be used to transmit the beam across an extremely thin air gap, and through the top of the recording medium to the recording layer. The SIL can be integrated with a flying magnetic head assembly. The air gap forms a bearing over which the flying head rides during operation. For near-field recording, the thickness of the air gap is less than one wavelength of the recording beam. Transmission of the beam is accomplished by a technique known as evanescent coupling.
For either substrate-incident or air-incident recording, the recording beam heats a localized area of the recording medium above its Curie temperature to form a magnetizable domain. The domain is allowed to cool in the presence of a magnetic field. The magnetic field overcomes the demagnetizing field of the perpendicular anisotropy recording medium, causing the localized domain to acquire a particular magnetization. The direction of the magnetic field and the resulting magnetization determine the data represented at the domain.
With beam modulation recording techniques, the magnetic field is maintained in a given direction for a period of time as the beam power is selectively modulated across the recording medium to achieve desired magnetizations at particular domains. According to magnetic field modulation (MFM) recording techniques, the beam is continuously scanned across the recording medium while the magnetic field is selectively modulated to achieve desired magnetization. Alternatively, the beam can be pulsed at a high frequency in coordination with modulation of the magnetic field.
To read the recorded data, the drive applies a lower intensity, plane-polarized read beam to the recording medium. Upon transmission through and/or reflection from the recording medium, the plane-polarized read beam experiences a Kerr rotation in polarization. The Kerr angle of rotation varies as a function of the magnetization of the localized area. An optical detector receives the read beam and translates the Kerr rotation angle into an appropriate bit value.
The amount of data storage capacity for a given magneto-optic disk depends on the spatial density of domains on the disk and the effective recording surface area of the disk. Greater spatial density results in more data per unit surface area. Greater recording surface area naturally results in greater storage capacity for a given spatial density. Recording surface area is limited, however, by disk size. Disk size has been limited in part by drive footprint requirements. Spatial density is limited primarily by the spot size of the drive laser. In other words, spatial density is a function of the ability of the drive to direct a beam to increasingly smaller domains in a consistent manner. Near-field, air-incident recording, in particular, has the potential to produce extremely small spot sizes using evanescent coupling and the resultant high numerical aperture, thereby providing increased spatial density and data storage capacity.